The present invention relates to a steam soak or cyclic steam stimulation (or huff and puff) process for recovering oil. More particularly, the invention relates to a process for increasing the oil-to-steam ratio in a steam soak operation in a relatively thick homogeneous reservoir which is susceptible to gravity override and contains substantially oil-desaturated zone in and above the portions of oil sand which are in fluid communication with the well.
Numerous patents have disclosed various uses of steam and surfactants in connection with steam soak oil recovery processes. For example, in 1966 U.S. Pat. No. 3,292,702 described a steam soak process in which aqueous surfactant was injected just before injecting steam in order to provide increased injectivity and more complete backflow of steam condensate. In 1967, U.S. Pat. No. 3,357,487 described a steam soak process in which a surfactant was injected "directly into the steam" for "increasing the sweep efficiency of the injected fluid" (Col. 3, lines 51-62). In 1968, U.S. Pat. No. 3,412,793 described a steam soak process which was said to "attain the known advantages of foam plugging of highly permeable earth strata, but additionally can control the length of time which those strata will remain plugged, so that they may again be subjected to steam drive or steam stimulation for any length of time desired." (Col. 2, lines 12-16); with the plugging being due to injecting a small amount of surface active agent directly into the steam line at the surface of the well. In 1976, U.S. Pat. No. 3,994,345 described a steam soak process in which "long shut-in periods for the well which may be for a period of about two weeks" (Col. 1, lines 58-60) are avoided by injecting steam then injecting "material which will cause to be formed in the formation a condensable foam blocking zone" (Col. 2, lines 5-7); such as a steam foam present in an amount and of a strength which is preferably sufficient to "block the passage of steam into the well until the steam has transferred to the formation substantially all of the heat" (Col. 4, lines 9-12). In 1978, Canadian Pat. No. 1,031,697 described a steam soak process for producing oil from a zone immediately underlying a gas cap by first plugging the gas cap with enough self-collapsing foam, e.g., a steam foam of the type described in U.S. Pat. No. 3,412,793, to keep steam from entering the plugged zone. In 1979, Canadian Pat. No. 1,057,648 described a process for increasing the backpressure of steam used in a steam soak process in which a thief zone is being plugged by steam foam of the type described in U.S. Pat. No. 3,412,793, by injecting alternating slugs of steam and surfactant to form the steam foam plug.
As far as applicants are aware, it appears that in steam soak processes in which a heavy viscous oil contains a substantially oil-desaturated zone that tends to contain, intake and/or retain significant amounts of steam and/or gas, the previously proposed methods for improving the efficiency of a steam soak operation were designed for plugging and blocking such a desaturated zone with a steam foam that is capable of preventing steam inflow or outflow until the foam collapses due to the cooling and condensing of the steam that forms the gas-phase of the foam.